Lithium secondary batteries are widely utilized in portable electronic equipment, personal computers, and the like. While miniaturization and weight reduction are required for the lithium secondary batteries, increasing the energy density is an important problem to be solved.
There are several methods for increasing the energy density of a lithium secondary battery, and among them, increasing the operating voltage of a battery is effective. A lithium secondary battery using lithium cobaltate or lithium manganate as a positive electrode active material has an average operating voltage of 3.6 to 3.8 V (4 V class) versus a metal lithium reference. This is because the operating voltage is defined by the oxidation-reduction reaction of cobalt ions or manganese ions (Co3+← →Co4+ or Mn3+← →Mn4+).
On the other hand, for example, a spinel compound in which a part of manganese in lithium manganate is replaced by nickel or the like, specifically LiNi0.5Mn1.5O4 or the like, shows a potential plateau in a region of 4.5 V or more. Therefore, by using these spinel compound as a positive electrode active material, 5 V class operating voltage can be achieved. In a positive electrode using the spinel compound, manganese is present in the tetravalent state, and the operating voltage of the battery is defined by the oxidation-reduction of Ni2+← →Ni4+ instead of the oxidation-reduction of Mn3+← →Mn4+.
LiNi0.5Mn1.5O4 has a capacity of 130 mAh/g or more and an average operating voltage of 4.6 V or more versus metal lithium, and has smaller lithium absorbing capacity than LiCoO2 but has higher energy density than LiCoO2. For such a reason, LiNi0.5Mn1.5O4 is promising as a positive electrode material.
However, in a battery using a high potential positive electrode active material, such as LiNi0.5Mn1.5O4, Li(LixNiyMn1-x-y)O2 (0.1<x<0.3, and 0.1<y<0.4), or LiCoPO4, the operating voltage is higher than in a battery using LiCoO2, LiMn2O4, or the like for a positive electrode active material, and on the other hand, a problem is that the decomposition reaction of the electrolytic solution proceeds easily in the contact portion between the positive electrode and the electrolytic solution, and the life shortens.
As techniques for improving the life of a battery, many examples of additives used in electrolytic solutions are reported. Many electrolytic solution additives form a film on the negative electrode to reduce the reactivity between the electrolytic solution solvent and the negative electrode and improve life. When a positive electrode operating at high voltage is used, the additive component remaining in the electrolytic solution reacts with the high potential positive electrode, which may cause the lowering of the capacity retention ratio and the generation of gas.
Patent Literature 1 describes a polymer electrolyte comprising a phosphate ester having ether group. Patent Literature 2 describes a nonaqueous solvent comprising a phosphate compound having a group comprising an ether bond. In addition, Patent Literature 3 describes a nonaqueous electrolytic solution comprising a phosphate ester having ether group and a halogen atom-substituted alkyl group.